This invention relates generally to the determination of blood clotting time, and more specifically concerns an apparatus for conveniently and reliably monitoring blood clotting time, suitable for home use, wherein the blood clotting time is used to monitor dosage levels of anticoagulant medication.
It has been estimated that over two million people in the United States are taking anti-coagulant medication, which is used to prevent either a possible first or a repeat stroke incident. The medication dose for a particular individual is selected to produce a prothrombin time (PT), i.e. clotting time, which is high enough to prevent a stroke, but not so high as to substantially prevent clotting, with the accompanying risk of hemorrhage. Dosages to accomplish these objectives vary significantly from patient to patient and even from time to time in the same patient due to dietary and other factors.
Besides the difficulty of maintaining a regular, accurate monitoring of prothrombin times to appropriately balance the blood clotting and hemorrhage risks, it has been established that incorrect levels of medication have significant undesirable side effects in most patients, including various skin effects, diarrhea, fever and nausea. Current medical practice typically prescribes a dose which is significantly below the actual indicated medical level to prevent stroke to avoid the other risks. Regular monitoring, i.e. on a daily basis, however, would likely permit a more appropriate dosage of medication to safeguard against stroke while still avoiding the hemorrhage risk, taking into account effects of diet, etc.
However, regular monitoring has not up to this point been practical due to the cost and inconvenience of repeated visits to a facility where blood is drawn followed by laboratory-testing. Current home-based devices are quite expensive and inconvenient to use on a regular basis, particularly by more elderly patients. One reason for this is the relatively large amount of blood which is required for a prothrombin time analysis by such devices.
Hence, an apparatus suitable for home use which is economical and accurate in establishing prothrombin time, yet requires relatively little blood for each test, would be highly desirable.
Accordingly, the present invention includes an apparatus for determining prothrombin time, suitable for home use, comprising: a compartment-like assembly which includes therein a reservoir for blood, the reservoir including a blood clotting agent therein; means, such as a needle element, for obtaining blood from a person being tested; means for moving the blood obtained from the person being tested to the blood reservoir; an ultrasound signal device for generating an ultrasound signal and transmitting it through the blood which has been moved into the reservoir when the reservoir contains enough blood to permit two-way propagation of the ultrasound signal but which has not begun to clot and also when the blood has substantially clotted, represented by a known increase in the two-way propagation (transmission) time; and processing means for determining the prothrombin time of the blood from the time it takes for the two-way propagation time to increase by a known amount.